1. Field of the Invention
The present invention relates to a power consumption reducing apparatus and method.
2. Description of the Related Art
Power management is one of the most important issues in all types of wireless network environments. This is because devices forming a network are battery-powered mobile terminals. In the prior art, in order to maximize the throughput of a network, power consumption has been ignored to some extent and only those products that work in a full-powered state have been developed. However, transmission of multimedia data is expected to grow exponentially in the near future, the transmission of this huge volume of multimedia data demands much faster transmission speed, and in line with the higher transmission speed, power consumption also increases in proportion to the speed. Accordingly, development of a low power protocol is more important than ever.
FIG. 1 is a format of an element traffic indication message (TIM) of a beacon frame.
A wireless network is broken down to an infrastructure network and an ad-hoc network. The infrastructure network, or the infra network, is constructed by expanding from an existing wire LAN to a wireless LAN after building access points (AP). The ad-hoc network refers to a LAN constructed only by wireless terminals. In the infra network, all stations in an inactive state moves into an active state temporarily in order to receive a beacon frame. Though the information in ‘partial virtual bitmap’ field of the TIM shown in FIG. 1, a station which receives a beacon frame checks whether or not an AP has data to be transmitted to the station. For example, if this field is set to 1, it means that the AP has buffered data. If through the field it is confirmed that the AP has buffered data, the station maintains the active state in order to receive the data. If it is confirmed that the AP does not have buffered data, the station sets network information and then returns to the inactive state such that power consumption is reduced. Meanwhile, since there is no AP in the ad-hoc network, power consumption is reduced by using an ATIM window and a management frame referred to as an ad-hoc traffic indication message (ATIM).
FIG. 2 is a format of an element independent basic service set (IBSS) of a beacon frame.
An ATIM window is an interval through which an active state of all stations is maintained and the interval is defined in the ATIM window field of the IBSS element included in a beacon frame.
FIG. 3 is a format of an ATIM message.
During an ATIM window interval, the management frame called an ATIM message is exchanged and the message is to indicate to a station in an inactive state that there is data to be transmitted by a source station. As shown in the ATIM message format of FIG. 3 (the ATIM message is a message in which the frame body part of an ordinary management frame is null), the frame has no special meaning and is only to indicate to a destination station that there is buffered data such that even after the ATIM window closes the active state of the station is maintained.
FIG. 4 is a waveform diagram of signals used in a power consumption reducing apparatus using the prior art ATM window.
With a beacon frame transmitted in each target beacon transmission time (TBTT), that is, a beacon frame transmission period, as a starting point, all stations receive network information and enter into an active state for a new setting, and at the same time an ATIM window begins in step 41.
A first station transmits an ATIM message to a second station during an ATIM window interval so as to inform the second station that there is buffered data in step 42. The second station informed of the buffered data remains in an active state until the next ATIM window closes in step 43. Accordingly, the first station can transmit data to the second station. A third station which does not receive an ATIM message returns to an inactive state if the ATIM window closes such that power consumption is reduced in step 44.
In transmission of an ATIM message, successful transmission is confirmed by whether or not an acknowledgment frame (ACK frame in IEEE 802.11a standard) is received. If an acknowledgment frame is not received, the transmission is regarded as a failure and after a time period allocated by a back-off timer by a back-off algorithm, retransmission is tried. If an ATIM message cannot be transmitted in a given ATIM window, it will be tried in the next ATIM window. ACK frame is not needed for an ATIM message that is multicasted. Since a medium is accessed through a distributed coordination function (DCF) method that is an IEEE 802.11 method, transmission of an ATIM message is based on channel contention. Accordingly, stations are using channels through contention and stations that fail in the contention have to wait for a time period allocated by a back-off timer and then contend again. If a given window closes, basically, stations return to an inactive state to reduce power consumption and only those stations which receive an ATIM message remain in an active state even when the ATIM window closes and receive transmitted data. If an ATIM window closes, a period for actual transmission of data remains as shown in steps 42 and 43. A station that receives an ATIM message can exchange data only in the transmission period for actual data transmission and remain in an active state until the next ATIM window closes. Generally, the ratio of an ATIM window and the actual data transmission period is about 1:4. Even in the actual data exchange period, the DCF method that is an IEEE 802.11 standard, is used and stations are contending for a channel. Accordingly, only those stations which win in the contention can transmit data through a selected channel and the loser stations have to wait for a predetermined time period by the back-off algorithm and again contend for a channel. If a station cannot transmit data in a given period due to continuous failure in the channel contention, the station can transmit data in a data transmission period of a next ATIM window.
Accordingly, in the power consumption reducing method using the prior art ATIM window, the frame payload of an ATIM message frame which is transmitted during an ATIM window interval has a null value and no data information. It is only to maintain the active state of a station that received an ATIM message and therefore is a clear network overhead.
FIG. 5 is a table showing a variety of modes of IEEE 802.11a physical layer.
Generally, in order to analyze the performance of a network, a network throughput is calculated. When calculating the throughput, management and control frames are not considered and only data frames corresponding to actual data are used for calculation.Throughput=MPDU payload length/periods used for successful data transmission  (1)
Here, MPDU stands for a Mac protocol data unit and indicates an actual data frame. When IEEE 802.11a is used, in order to identify the duration for data transmission, the number of data symbols should be identified. If an obtained number of symbols is multiplied by 4 μs that is an interval between symbols used in WEEE 802.11a, the length can be identified.Number of symbols=Ceiling((16+8*MPDU length+6)/BPS)  (2)
BPS, or bits per second, of Equation 2 can be obtained using the table of FIG. 5. Assuming the data rate is 54 Mbps and all conditions are idealistic, if the network throughput is obtained using the Equation 1, the maximum throughput can be obtained as up to 30.8087 Mbps.
                                                                                                                                                                                      Average                          ⁢                                                                                                          ⁢                          back                          ⁢                                                      -                                                    ⁢                          off                                                ⁢                                                                                                                                                                                                                          interval                        ⁡                                                  (                          I                          )                                                                                                                    =                                ⁢                                                                            (                                                                                                    2                            ^                            I                                                    *                                                      (                                                                                          CW                                ⁢                                                                                                                                  ⁢                                min                                                            +                              1                                                        )                                                                          -                        1                                            )                                        /                    2                                    *                  Slot                  ⁢                                                                          ⁢                  Time                                            ,                                                                                        ⁢                              0                <=                I                <=                6                                                                                                        =                                ⁢                                                                            (                                              CW                        ⁢                                                                                                  ⁢                        max                                            )                                        /                    2                                    *                  SlotTime                                            ,                            ⁢                              I                >=                6                                                                        (        3        )            
Here, I denotes the number of continuous retransmission due to the failure in channel contention.
One of the most ordinary factors affecting the throughput of a network is transmission delay due to an occurrence of an error. A representative case can be a back-off timer and retransmission. First, as for a back-off timer, whenever there is a failure in a contention, a back-off interval is newly given. Using the Equation 3, the back-off interval due to each failure is calculated. When the maximum throughput is 30.8087 Mbps that is idealistic, network contentions should not be considered and calculation is performed using the average back-off interval value that corresponds to a time when a back-off interval is not given. That is, in the Equation 3, value I is set to 0 and then calculation is performed. When there is one contention, that is, when value I is set to 1, the throughput obtained by the calculation decreases by 4.8065 Mbps, a 15.60% decrease from the maximum throughput. As for retransmission, when 10 frames are desired to be transmitted, and if an error occurs in one frame among the frames and 10% of the entire amount is retransmitted, the throughput decreases by 2.5035 Mbps, an 8.13% decrease from the maximum throughput. Accordingly, it can be shown that one of the most important factors affecting the network throughput is the back-off interval.
Therefore, in the power consumption reducing method using the prior art ATIM window, since this back-off interval is used in two places, including an ATIM window and a data transmission period, the network throughput is affected as much.